Pump and pressure tank for low volume water connection

[Estrogen Hostage]

I have a large house in the country with six bathrooms, and irrigation system, and eight of us living here. I have a city water connection that has a small meter. I have acceptable static water pressure but once I begin drawing higher demand the pressure drops to an unacceptable level. I have been told that a pressure tank will help with this but I would rather put a pump on to ensure that I can take a shower while doing laundry, irrigating the lawn, etc.

my question is about the pump set up. All of the advice I had found is to put a plastic tank in the basement and use a conventional jet pump so that I don’t put a suction on the incoming line. The second point is important to me because I have a small cabin and other outdoor irrigation and livestock watering attached to the water connection upstream of my house.

The part I don’t understand is about the tank before the pump. Putting in an unpressurized plastic tank and filling with the float valve seems like a good way to introduce contamination, so why not put in a similar sized pressurized air over water tank? It seems if I filled the tank in one port at the top and discharge from the bottom it would effectively Decouple the two systems, and at the times when the pump volume is higher than the meter flow the air pressure will just let the water level drop in the tank. Thing is that I can find no information about such a set up online. I already own the pump and bladder tank but I don’t really want to spend money not knowing if such a system will work like I’m expecting.

I believe the biggest risk would be that the demand could still be greater than the meter flow but I’m sure there is some type of switch I could put in the tank to shut the pump off if it runs all the way down.

 

[Valveman]

If the demand is greater than the supply from the meter, a storage tank system is necessary. It would look like this except being fed by a float valve from a city line instead of the well pump as shown. You can use a "pump down" float switch in the bottom of the tank to keep the booster pump from coming on when there is low water in the storage tank. A pressure tank only holds 25% water, so it would take a huge one to act as storage for the pump to draw from. It would work to a point. But if you used all the water in a pressurized storage tank you would be drawing more than the meter supplied as you are trying not to do. This system shows a Cycle Sensor installed, which will also protect the pump from running dry and doesn't need a float switch.

 

[Sponsor]

 

[Reach4]

I have been told that a pressure tank will help

Putting in an unpressurized plastic tank and filling with the float valve seems like a good way to introduce contamination, so why not put in a similar sized pressurized air over water tank?

I think whatever you could do with an air over water tank, you could do with a pre-charged diaphragm tank. Plus, you would not have to introduce air.

 

[Valveman]

When a pressure tank of any kind is empty of water it is no different than a pipe plug. The pump would be drawing directly from a meter that is too small to supply the pump and/or demand. Plus, when boosting public water it is usually chlorinated, and will last quite a while in a storage tank.

 

[Estrogen Hostage]

If the demand is greater than the supply from the meter, a storage tank system is necessary. It would look like this except being fed by a float valve from a city line instead of the well pump as shown. You can use a "pump down" float switch in the bottom of the tank to keep the booster pump from coming on when there is low water in the storage tank. A pressure tank only holds 25% water, so it would take a huge one to act as storage for the pump to draw from. It would work to a point. But if you used all the water in a pressurized storage tank you would be drawing more than the meter supplied as you are trying not to do. This system shows a Cycle Sensor installed, which will also protect the pump from running dry and doesn't need a float switch.
View attachment 58952

So I guess this is the point that I’m getting stuck on. I know from experience filling up hot water heaters and such that if I take ian unpressurized tank and begin filling it with water that The air will pressurize and the tank will fill a good amount of the way with water. I know it’s not 100% but it’s not 25% either. I’ve got the idea in my head that says 60 PSI is about 4 atm, The resulting tank pressure would equalize with a water column will be 80%. Feel free to correct me if I’m wrong.

I also know from experience that the pump will pump it all the way down to zero psi, and even create a suction on the bottom of the tank. So if I put a 100 gallon air tank in and let it fill with 80 gallons of water, why am I repeatedly being told that I only get 25% of the tank volume available?

 

[Estrogen Hostage]

I think whatever you could do with an air over water tank, you could do with a pre-charged diaphragm tank. Plus, you would not have to introduce air.

conceptually this makes sense to me if the drawdown is from 30 to 50 psi because you can essentially empty the tank and still have the 30 psi precharge present. To me it seems that a pressurized air over water tank would have much greater capacity at a lower cost. Plus I have been told that putting a diaphragm tank at the inlet of a pump could damage of the tank.

 

[Valveman]

A water heater doesn't have any air in it. If it is a 50 gallon water heater it will hold 50 gallons of water. But the only reason a diaphragm style pressure tank will hold 25% water is because it is pre-chraged with air. 40/60 switch needs 38 PSI pre-charge of air in the tank to get out 25% water. The less air you put in the tank, the more water it will take, but the less water you will get out of it before the pressure falls off to zero. When the pressure falls to zero it will be like putting your finger over a straw full of ice tea. There will be no more water coming out of the straw and the pump will pull a vacuum on the meter if it is too small to supply the water needed.

 

[Estrogen Hostage]

A water heater doesn't have any air in it. If it is a 50 gallon water heater it will hold 50 gallons of water. But the only reason a diaphragm style pressure tank will hold 25% water is because it is pre-chraged with air. 40/60 switch needs 38 PSI pre-charge of air in the tank to get out 25% water. The less air you put in the tank, the more water it will take, but the less water you will get out of it before the pressure falls off to zero. When the pressure falls to zero it will be like putting your finger over a straw full of ice tea. There will be no more water coming out of the straw and the pump will pull a vacuum on the meter if it is too small to supply the water needed.

Right, but I don’t need pressure. That’s what the pump is for. The tank is to provide volume.

The comment about a water heater was made in regards to installing a new one. I know that unless I have a valve on the tank will fill most of the way with water and have a little air at the top.

 

[Reach4]

conceptually this makes sense to me if the drawdown is from 30 to 50 psi because you can essentially empty the tank and still have the 30 psi precharge present. To me it seems that a pressurized air over water tank would have much greater capacity at a lower cost.

You are right, if you can let the pressure drop to near zero in that tank. If you could pressurize the tank with about 3 atmospheres of water pressure and able to suck down to zero psi, you could approach 75%. This brings up two difficult questions:

1. How do you shut down the pressure pump before the water is gone from the tank and you are ready to suck air?
2. How do you keep the right amount of air in the tank? Air gets absorbed into the water, and needs to be replenished -- but not overdone.

It is possible you could use a big tank designed as a RO pressure tank. Those have diaphragms/bladders that are made to have low precharge and to expand with water more than those made as well pump pressure tanks.

 

[Valveman]

Without a pre-charge you can force a lot of water in a tank, and a pump can suck a lot of water out. But with a standard tank the pump will also suck the air out, or with a diaphragm will hit the bottom and plug off the hole. Then you are either pumping air and/or drawing more than the meter can supply. Drawing a vacuum on a pressure tank is not good.

 

[Estrogen Hostage]

I’ll be honest. I haven’t spent a great deal of time specifying components. I already own the pump and would prefer to use it if possible but I’m not hung up on it. I very strongly like the idea of using a pressurized tank that is a sealed system instead of a plastic tank with a float switch. The cycle stop valve mentioned above seems like it would be a good addition because it would not affect the pressure upstream as much when the pump comes on. I have the idea that about 150 gallons of useable water storage would be sufficient, so a 250 to 300 gallon tank would be what I would need. I have found a pretty reasonably priced flow tech 119 gallon tank for about $400 but I have not yet found a tank about twice that size. I will look into the ones you mentioned.

in regards to your questions, I have not yet considered the second point and have not found a solution to the first yet. I feel like I can’t be the first person to want to know the volume of water in a pressurized tank so I need to look for some type of float switch or sensor. do you think that one of the air maker valves can be used with my pump?

 

[Valveman]

Just be sure to use a vacuum breaker so the pump sucks air instead of making the tank look like the one above.

 

[Reach4]

You might measure how many gpm your supply can deliver with an open valve. If 7 gpm or more, a pressure pump with no storage might do fine. If it is 10, that would be really good.

 

[Estrogen Hostage]

I don’t have any good way to open up the flow in a single location where I could measure how many GPM. By opening up several shower valves in the house I was able to get a flow of six gallons per minute at 35 psi coming in. I have the idea that if I could open up the flow at a single valve it might even be 10 or 15 GPM at low psi. Static pressure was 55 psi. The problem is that this was measured at the lowest point in the basement, before the pressure drop in the water softener and the pressure drop in the pipes and about 35 feet of vertical rise to the highest valve in the house. That showerhead has enough pressure when it’s the only thing on but if you get very many things going at the same time like the sprinklers or the water softener regen or a few showerheads there are very big pressure problems upstairs.

The way I see it any pump I select is either too small to feed the house or will have the ability to outstrip my supply.

 

[Valveman]

The way I see it any pump I select is either too small to feed the house or will have the ability to outstrip my supply.

Not necessarily. You can use a 20 GPM pump with a CSV and small tank. Then the pump will only supply as much as you are using. As long as you don't use more than say 12 GPM, the CSV will make the pump act like a 12 GPM pump and you won't outstrip your supply.

 

[Estrogen Hostage]

Based on my understanding, it's still drawing full volume when it first comes on. Your valve will help but not solve my problem, IMHO.

 

[Valveman]

Based on my understanding, it's still drawing full volume when it first comes on. Your valve will help but not solve my problem, IMHO.

Setting the CSV the same pressure as the pressure switch on setting the pump will startup producing only the amount being used. The 4.5 gallon size tank only holds 1 gallon of water, which will give you 1 minute of run time after the faucets are closed. Even if you set the CSV at 50 in the middle of the 40/60 pressure switch setting, it would only pump 20 GPM for 2 seconds before it put a 1/2 gallon of water in the tank and the CSV shut down to the amount being used. Even that is not going to draw more than your meter can supply. But setting the CSV the same as the pressure switch on setting there is no way for the pump to draw more than you are using at any given time.

 

[Estrogen Hostage]

Fair enough - I hadn't considered that. I suppose I could install the existing pressure tank before the pump to COMPLETELY eliminate the risk of creating a problem upstream every time the pump shuts on.

Here is the pump curve for the pump I already have. Clearly it's too big for the meter and I agree with your points. Let's say I want to run this at about 65PSI nominal (so maybe 55PSI-75PSI) and I want to design this so that I CANNOT put negative pressure on the incoming pipe. As you said the only way to do that is by controlling the load. I'm not comfortable with that caveat because I use float waterers for livestock upstream and there is a risk of contamination there.

Can you think of a way to do this, perhaps with some type of pressure switch that opens a set of contacts at 10PSI on the incoming water pressure? That way I just shut my pump off. I can live with that outcome.

EDIT: like this maybe: https://www.freshwatersystems.com/products/low-pressure-shut-off-switch-1-8-male-npt

 

[Valveman]

You can use a loss of prime pressure switch on the suction line. However, they tend to bounce the pump on/off as the incoming pressure comes back and then can't keep up with the pump. An easy way with less nuisance trips is to use a vacuum breaker on the suction line, and a Cycle Sensor to shut the pump off if that vacuum breaker opens and let air in the lines. The vacuum breaker would also prevent stock tanks from draining back into the system.

 

[Estrogen Hostage]

Fair point...I hadn't considered that. I'm thinking a timer to prevent the motor restarting for say 30-60 seconds would be a better way to accomplish that. This would have the benefit of giving the occupants of the house time to adjust their consumption while managing their frustration with the situation.

I did use a vacuum breaker on the hydrants but would rather not need to. These components are inexpensive anyway, better to design things to be safe and not rely on my wife or kids to remember what I tell them at the risk of getting us all sick!